Two Questions; engine exhaust heat exchanger and pressurised storage tanks

[Dune]

Some of you may recall I am installing a diesel genset for co-gen using vegtable oil. I need to design and build a heat exchanger to extract heat from the exhaust gases. My first idea is to simply run a section of the exhaust pipe through a length of larger pipe, a water jacket. I am wondering if this would be efective enough.
My other question regards two 200 gallon tanks I found. They are rated at 200 p.s.i. I was thinking of cutting the ends off, rolling another tube section and welding them back together to form two 400 gallon tanks, which would be a reasonable though still small amount of storage. Obviously I am a professional welder with over thirty years experience. Any input is welcome. Thankyou.

 

[deerefanatic]

Well, I would advise extreme caution with welding together two pressure vessels. But, if you plan to put them together end to end, I'd weld them together along the seem, then wrap that with 8"x1/4" stock and weld again. Use the highest heat setting you can get away with while not burning through and use a MIG welder so you can get it water tight....... I'd check for leaks after welding the two tanks together before wrapping the seem with the 1/4 stock.

As for the heat exchanger, a hot water air core would be ideal.... Something like a 12x12 hot water air core in a box that the exhaust can pass through. you'd get efficient heat transfer, and not too much restriction. I'd put it as close to the engine/turbo as possible and leave off the muffler until after the heat exchanger (mufflers just cool exhaust......) Be sure to leave a way to clean the heat exchanger as that diesel will soot it up.

 

[Gooserider]

Noting that you are from MA, I would advise that you be careful about checking rules on pressure vessels - I believe the current boiler codes require ALL pressure vessels in a heating system to be ASME stamped - something hard to get w/ a home brew tank... (It is one reason I'm planning non-pressure storage, as it avoids that entire set of hassles...)

In terms of the welding otherwise, you know your skills better than we do, I don't quite see how you are planning to turn two 200g tanks into two 400g tanks, but if you are doing so by adding metal, why limit yourself to just that size?

For the heat exchanger, a water jacket setup should work if it is long enough (not sure how long that is...) - I would also suggest that keeping the thickness of the water jacket gap fairly narrow will help, along with adding turbulators of some sort to both the exhaust and water sides, and to be sure to plumb it "counterflow" style. If you want to shrink the overall length, it might be good to split the exhaust flow into multiple smaller pipes...

Gooserider

 

[jebatty]

I take it you are well aware of the need to allow proper exhaust flow from the engine. Adding a hx may increase back pressure and affect engine operation.

 

[Nofossil]

For the pressure tanks, I'd check first to see if you can find used propane tanks. They're sometimes available for really low prices - perhaps less than the metal that you'd need to do it yourself. You could then apply your time and energy into welding up inlet and outlet ports that maximize stratification.

For exhaust heat exchangers, I think you'll be disappointed. Heat transfer is a function of surface area and temperature difference. I expect the exhaust isn't as hot as you might want, and it's hard to get very many square feet of surface area. The soot problem rules out using an exchanger with small air passages (such as an automotive radiator or heater core). I suspect that the best you're likely to find is a boiler type heat exchanger - several parallel steel pipes in a water jacket.

 

[deerefanatic]

Speaking of a boiler type heat exchanger, I have the jacket off of my old homemade gasifier. It's sitting on the scrap trailer right this moment. 10- 2" pipes, 48" long through a water jacket. It's a heavy sucker though. but all welded up and no leaks. Contact me if you'd like to buy it.

 

[Dune]

For the pressure tanks, I'd check first to see if you can find used propane tanks. They're sometimes available for really low prices - perhaps less than the metal that you'd need to do it yourself. You could then apply your time and energy into welding up inlet and outlet ports that maximize stratification.

For exhaust heat exchangers, I think you'll be disappointed. Heat transfer is a function of surface area and temperature difference. I expect the exhaust isn't as hot as you might want, and it's hard to get very many square feet of surface area. The soot problem rules out using an exchanger with small air passages (such as an automotive radiator or heater core). I suspect that the best you're likely to find is a boiler type heat exchanger - several parallel steel pipes in a water jacket.

The local propane supplier has some serious liability worries. At one point, they were available, but now they get angry just at being asked.

A diesel (naturaly aspirated) runs exhaust temps between 600-750 degrees F when under full load. As my system will be grid-connected, it will be fully loaded always. Surely this is enough temperature difference to make this worthwhile?

 

[Dune]

Speaking of a boiler type heat exchanger, I have the jacket off of my old homemade gasifier. It's sitting on the scrap trailer right this moment. 10- 2" pipes, 48" long through a water jacket. It's a heavy sucker though. but all welded up and no leaks. Contact me if you'd like to buy it.

My ehaust pipe is only 2". Seems your unit might be a bit oversized?

 

[Dune]

I take it you are well aware of the need to allow proper exhaust flow from the engine. Adding a hx may increase back pressure and affect engine operation.

Water jacket on outside of pipe should not cause interference.

 

[Dune]

Noting that you are from MA, I would advise that you be careful about checking rules on pressure vessels - I believe the current boiler codes require ALL pressure vessels in a heating system to be ASME stamped - something hard to get w/ a home brew tank... (It is one reason I'm planning non-pressure storage, as it avoids that entire set of hassles...)

In terms of the welding otherwise, you know your skills better than we do, I don't quite see how you are planning to turn two 200g tanks into two 400g tanks, but if you are doing so by adding metal, why limit yourself to just that size?

For the heat exchanger, a water jacket setup should work if it is long enough (not sure how long that is...) - I would also suggest that keeping the thickness of the water jacket gap fairly narrow will help, along with adding turbulators of some sort to both the exhaust and water sides, and to be sure to plumb it "counterflow" style. If you want to shrink the overall length, it might be good to split the exhaust flow into multiple smaller pipes...

Gooserider

Thanks for the input. I will consider this carefully.

 

[Dune]

So now I am thinking I will go with a simple pipe within pipe water jacket, most likely copper pipe. Any input as to how to calculate the lenght needed?

 

[Dune]

consider the exhaust pipe being held rigid as the engine vibrates which could result in a cracked exhaust manifold! otherwize good luck

I am using an exhaust flex, a foot of staniless steel flexible material betwwen the exhaust manifold and the piping. I have built dozens of exhaust systems for diesel engines in commercial boats. Thanks for your thoughtful comment.

 

[deerefanatic]

My unit may be oversized.. But, oversized will slow the exhaust flow and therefore extract more heat while posing less restriction.... Just a thought.

 

[Dune]

My unit may be oversized.. But, oversized will slow the exhaust flow and therefore extract more heat while posing less restriction.... Just a thought.

Feel like posting a picture of it or e-mailing it to [email protected]

 

[TMonter]

IF you're going to use a diesel heat exchanger also be careful on material selection, anything other than 304 stainless or higher will have problems with the sulfur and other corrosive properties of diesel or bio diesel. You're going to need a fair amount of surface area to get a reasonable energy capture, so much in fact it may become cost prohibitive.

As for welding two vessels together that's a big no-no unless you have someone run stress and corrosion allowance calcs on the new design. Pressure vessels carry an ASME stamp precisely to prevent explosion risks associated with improper design and welding. Regardsless of what you do with the pressure vessel make sure you have a properly operating and sized safety relief attached to the vessel.

At the very least pick up a copy of the pressure vessel handbook and follow the design guidelines in it if you insist on building your own pressure vessel.

http://www.amazon.com/Pressure-Vessel-Handbook-Eugene-Megyesy/dp/091445823X

You might look into local industrial plants and mills as well that have shut down. Sometimes you can pick up a pressure tank for a song.

 

[Dune]

Thanks T. I think I will use the tanks as they are, and wait for more to apear. The Cape is not a very industrial place you know. The two tanks I found so far are coming from New York. As to the heat exchanger, I notice you recomend 304 S.S. The thermal conduction rate of stainless is very poor compared to copper, which is why I thought I would use copper. Do you think that copper would not be a good choice? I planned to TIG weld the unit with copper filler wire so there would be no issues with overheating solder joints. In marine engineering, copper heat exchangers for diesels are not uncommon, and usualy last quite a few years.

 

[deerefanatic]

Well, two things on the stainless and sulfur content.

1: With the ultra low sulfur fuel in use today, that is basically nothing to worry about.
2: He's burning veggie oil, which has no sulfur whatsoever in it.......

I'll get you some pics of my HX asap.

 

[TMonter]

Well, two things on the stainless and sulfur content.

1: With the ultra low sulfur fuel in use today, that is basically nothing to worry about.
2: He’s burning veggie oil, which has no sulfur whatsoever in it.......

I’ll get you some pics of my HX asap.

You're likely right, I've just seen so many rotted heat exchangers and air heaters over the years from improper material selection I'm pretty conservative when it comes to my material selection. Even with ULSD at 15PPM that is enough sulfur to cause problems depending on the number of operation hours used.